TUCSON – Mexican authorities uncovered another tunnel in Nogales, Sonora – across the border from Nogales, Arizona – which they suspect was used to smuggle drugs and people across the U.S.-Mexico border.

It's the third time they have made such a discovery in less than a month, and it comes amid President Donald Trump's demand for funding to build more physical barriers along the U.S.-Mexico border.

Mexican Federal Police on Wednesday posted a short video of the tunnel on Twitter. Police said the tunnel measured about 32 feet in length but offered few other details, such as the location or how they found it.

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In this March 6, 2017, photo, a member of the Border Patrol's Border Tunnel Entry Team walks in a tunnel in San Diego that spans the border between the California city and Tijuana, Mexico. Team members are known in the Border Patrol as "tunnel rats" — agents who go in the clandestine passages that have proliferated on the U.S.-Mexico border over the last 20 years to smuggle drugs. Associated Press

A member of the Border Patrol's Border Tunnel Entry Team looks up as he descends an entrance carved out by the Border Patrol leading to a tunnel spanning the border between San Diego and Tijuana, Mexico. Authorities have discovered more than 200 cross-border tunnels originating in Mexico since 1990, most of which entered the United States. Associated Press

A member of the Border Patrol's Border Tunnel Entry Team lowers a cable into a tunnel entrance in between two border barriers separating San Diego and Tijuana, Mexico. Authorities have discovered more than 200 cross-border tunnels originating in Mexico since 1990, most of which entered the United States. Many are shallow holes but some are elaborately constructed with hydraulic lifts, water pumps and rail cars. Associated Press

Members of the Border Patrol's Border Tunnel Entry Team work in a tunnel spanning the border between San Diego and Tijuana, Mexico. Above, a ventilation shaft created by the Border Patrol reaches to the surface. Associated Press

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The video shows two police officers opening up a manhole underneath the extensive drainage channels that lie below the downtown areas in the twin cities of Nogales. The channels are used to ease the flow of runoff water during storms and typically flow north from Sonora into Arizona because of the terrain. But they are also commonly used in smuggling attempts.

As the video progresses, it shows the officers inside the tunnel, following it to the point where it originates at an unknown location. The officer recording is heard asking, "Is that the exit?" A second officer responds "yes" as he pushes open the entrance to the tunnel overhead.

Attempts to contact and request more information from the Mexican Federal Police were unsuccessful. U.S. Customs and Border Protection officials in Arizona are also unable to provide information because they furloughed their communication staff due to the ongoing partial government shutdown, which is now the longest in U.S. history.

Effectiveness of walls under scrutiny

Trump's insistence on border-security funding is central to the fight that has resulted in the shutdown.

As part of his push to secure more funding, Trump cites high numbers of drugs coming across the Southwest U.S. border and claims that "These numbers will be DRASTICALLY REDUCED if we have a Wall!"

We lose 300 Americans a week, 90% of which comes through the Southern Border. These numbers will be DRASTICALLY REDUCED if we have a Wall!

However, the vast majority of the drugs that he alludes to, including heroin, are increasingly caught at the legal ports of entry, which would be unaffected by the construction of additional physical barriers.

Critics routinely point to drug tunnels as a sign that walls don't work.

U.S. House Speaker Nancy Pelosi, D-Calif., broached the topic during one of several failed meetings between congressional Democratic leadership and Trump to try to end the stalemate.

As the president visited south Texas on Thursday, one of the border officials he met with explained how smugglers had turned to tunneling underneath areas of the border where physical barriers had been built.

"This is the second tunnel recently that we've located. This is an area that we actually have wall," Melissa Lucio, the Border Patrol's agent in charge of the McAllen station, told Trump as he looked on.

"We're doing such a great job in utilizing the right resources in that particular area, that they've (smugglers) become so frustrated they're using other tactics," she added. "They're actually digging tunnels. This is about 25 feet long, and 2 to 3 feet high."

Smugglers increasingly using tunnels

In this March 6, 2017 photo, a member of the Border Patrol's Border Tunnel Entry Team enters a tunnel spanning the border between San Diego and Tijuana, Mexico, in San Diego. They are known in the Border Patrol as "tunnel rats" - agents who go in clandestine passages that have proliferated on the U.S.-Mexico border over the last 20 years to smuggle drugs. (AP Photo/Gregory Bull)(Photo: Associated Press)

Tunnels have become an increasingly common tactic that smugglers have used to get around stricter enforcement at the border, especially in sections that have had physical barriers in place for many years and decades, such as Arizona and California's borders with Mexico.

Along the Arizona border, Nogales is one of the main areas where large numbers of drug tunnels are uncovered on either side.

Just last week, Mexican Federal Police located a 65-foot tunnel inside an abandoned business in Nogales, Sonora, about four blocks south of the international boundary, according to Mexican media.

Before then, on Dec. 17, Customs and Border Protection officials in Arizona said they and Mexican officials had located another tunnel in downtown Nogales as part of a "routine binational tunnel sweep."